Process for manufacture of composite semiconductor devices

ABSTRACT

An integrated semiconductor device is formed from two fabricated semiconductor devices each having a substrate by placing an etch-resist on the substrate of the one semiconductor device, by bonding the conductors of one of the fabricated semiconductor devices to the conductors of the other fabricated semiconductor device, flowing an uncured cement (e.g. epoxy) between the etch-resist and the other substrate, allowing the cement to solidify, and removing the substrate from the one of the semiconductor devices. More specifically, a hybrid semiconductor device is formed from a GaAs/AlGaAs multiple quantum well modulator having a substrate and an IC chip having a substrate by placing an etch resist on the modulator substrate, bonding the conductors of the modulator to the conductors of the chip, wicking an uncured epoxy between the modulators and the chip, allowing the epoxy to cure, and removing the substrate from the modulator.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.08/366,864 filed Dec. 30, 1994, U.S. Pat. No. 5,778,162, which is acontinuation-in-part of divisional U.S. application Ser. No. 08/236,307,filed May 2, 1994, abandoned, and of U.S. application Ser. No. 083,742,filed Jun. 25, 1993, now U.S. Pat. No. 5,385,632, all assigned to thesame assignee as this application.

FIELD OF THE INVENTION

This invention relates to bonding of fully-fabricated semiconductordevices onto other fully-fabricated semiconductor devices so as toproduce integrated units, and particularly to bonding fully-fabricatedphotonic elements, such as GaAs/AlGaAs multiple quantum well (MQW)modulators, onto fully-fabricated integrated circuit (IC) chips such asSi or even GaAs.

BACKGROUND OF THE INVENTION

Integration of photonic devices with silicon IC chips makes it possibleto combine the advantages of each. Among photonic devices, GaAs/AlGaAsmultiple quantum well (MQW) modulators are particularly beneficial asinput/output (I/O) elements on IC chips because they have a highabsorption coefficient of light and can serve as both receivers andtransmitters. They typically operate at an optical wavelength (λ) of 850nm (nanometers).

The aforementioned applications Ser. No. 083,784, filed Jun. 25, 1993,Ser. No. 08/236,307, filed May 2, 1994, and Ser. No. 08/366,864 filedDec. 30, 1995, disclosed methods of bonding a photonic element, such asa GaAs/AlGaAs (MQW) modulator, to an IC chip by bonding their respectiveterminals to each other, filling the interiors between them withphotoresist or a photoresisting cement, and then removing the substrateof the photonic device. Ser. No. 08/366,864 specifically discloseswicking a cement, such as epoxy into the interstices in place of thephotoresist. However, voids that extent to one or both substratessometimes form in the cement. These may allow that etch that removes thesubstrate on the modulator to affect other parts of the combined unit.

SUMMARY OF THE INVENTION

An embodiment of the invention involves confining the surface of thesubstrate with an etch-resist before bonding the conductors of onefabricated semiconductor device having a substrate to the conductors onanother fabricated semiconductor device having a substrate, flowing anuncured cement between the devices, allowing the etch-resist tosolidify, and removing the substrate from one of the semiconductordevices.

The various feature of novelty which characterize the invention arepointed out in the claims. Objects and advantages of the invention willbecome evident from the following detailed description when read inlight of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating a photonic device in theform of an MQW modulator containing a multiple quantum well modulatorunit.

FIG. 2 is a cross-sectional view illustrating an arrangement in a stepfor forming a device that integrates the multiple quantum well modulatorwith an integrated circuit chip according to an embodiment of theinvention.

FIG. 3 is a cross-sectional view illustrating an arrangement in anotherstep for forming a device integrating the multiple quantum wellmodulator with an integrated circuit chip according to an embodiment ofthe invention.

FIG. 4 is a cross-sectional view illustrating a device integrating aphotonic element with an electronic element according to an embodimentand embodying aspects of the invention.

FIG. 5 is a cross-sectional view illustrating a device integrating anumber of photonic elements on an IC and arranged according to anembodiment of the invention.

FIG. 6 is a plan view illustrating a device integrating an array ofphotonic elements on an IC according to an embodiment of the invention.

FIG. 7 is a schematic diagram of a system illustrating an arrangementfor performing steps in the manufacture of the device according toaspects of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1-4 illustrate a GaAs/AlGaAs 850 nm λ multiple quantum wellmodulator MOD, and a solder-bonding technique for integrating theGaAs/AlGaAs 850 nm λ modulator with an IC to form the device embodyingthe invention. FIG. 1 illustrates a multi-strata multiple quantum wellmodulator MOD arranged, according to an embodiment of the invention, forbonding to contacts on a Si device according to the invention.

In the modulator MOD, a GaAs substrate SUB supports a 1.5 μm layer LA1of n (i.e. n-doped) (10¹⁸ cm⁻³) Al₀.3 Ga₀.7 As grown on the substrateSUB. A 100 Å i (i.e. intrinsic) Al₀.3 Ga₀.7 As spacer SP1 on the layerLA1 spaces the latter from an i multiple quantum well modulator unit MQWcomposed of 55 periods of 90 Å GaAS wells and 30 Å Al₀.3 Ga₀.7 Asbarriers. A 70 Åi Al₀.3 Ga₀.7 As spacer SP2 on the multiple quantum wellmodulator unit MQW spaces the latter from a 500 Å p (i.e. p-doped) (10¹⁸cm⁻³) Al_(x) Ga_(1-x) As layer LA2 graded from X=0.3 to x=0, on thespacer SP2. A 500 Å p⁺ (5*10¹⁸ cm⁻³) GaAs layer LA3 covers the layerLA2.

The modulator MOD, at the substrate SUB, forms a 5 mm square piece andhas 110×110 μm gold p contacts CG (1000 Å thick) deposited on the layerLA3. The strata MQW, SP2, LA2, LA3, and CG form a 130×130 μm inner mesaME that extends to within 1500 Å of the n layer LA1. A 50×120 μm, 7000 Åthick, AuGe/Au n contact CO on the n layer LA1 extends upwardly to makeits top coplanar with the gold p contact CG. 4000 Å In caps COI1 andCOI2 cover respective contacts CG and CO. Redundant etch-resist layerRL1 covers the substrate SUB and the strata of the modulator unit MQW.

Manufacture of the modulator MOD is described in detail in theaforementioned applications Ser. No. 083,742 filed Jun. 25, 1993, Ser.No. 08/236,307 filed May 24, 1995 and Ser. No. 08/366,864 filed Dec. 30,1995. In the present embodiment for manufacture of the modulator in FIG.1, the procedure in part concludes with deposition of the resist layerRL1.

FIG. 2 illustrates the modulator MOD of FIG. 1 upside down in positionabove a portion of a Si device SD, such as an IC chip, as a step information of the integrated hybrid device of this embodiment. In FIG. 2the device SD includes a 1 cm square p type Si substrate SIS with Alcontacts COA1 and COA2 of the same size and spacing as the p and ncontacts CG and CO on the modulator MOD. These Al contacts COA1 and COA2are set to extend out of the page of FIG. 2 so that they would beexposed when the hybridization process is completed according to anembodiment of the invention. Indium contacts CI1 and CI2 on the Alcontacts also have the same size and spacing as the modulator contactsCG and CO.

To integrate the modulator MOD with an IC chip, the following occurs:

Patterning a 1 cm square p type Si substrate SIS with Al contacts COA1and COA2 of the same size and spacing as the p and n contacts CG and COon the modulator MOD. These Al contacts COAL and COA2 are set to extendout of the page of FIG. 2 so that they would be exposed upon completionof the hybridization process according to an embodiment of theinvention.

Depositing indium contacts COI1 and COI2 on the Al with the same sizeand spacing of the modulator contacts CG and CO.

Placing the modulator MOD upside down on the Si piece and aligning it.According to an embodiment of the invention, a precision controlleraligns the modulator MOD on the Si device SD.

FIG. 3 shows the modulator MOD on the Si device SD with the In contactsCOI1 and COI2 bonded to the contacts CI1 and CI2. Here cement CM1surrounds the contacts CG, CO, COI1, COI2, COA1, COA2, CI1 and CI2 belowthe layer RL1. The structure in FIG. 3 is achieved by the followingsteps.

Heating the unit to 200° C. for 15 minutes to melt the indium contactsinto each other. At this point the resulting unit is relatively stable(i.e., shaking does not cause it to break apart).

Wicking liquid cement CM1, such as epoxy, between the modulator MOD andthe Si device SD by depositing drops of cement on the Si substrate aboutthe GaAs/AlGaAs modulator MOD and allowing it to flow against its edge.The cement CM1 flows between the layer RL1 and the Si device SD.

Curing the cement CM1 as need. The dried cement CM1 serves two purposes.First, it protects the modulator MOD during substrate etching. Second,it provides additional mechanical support.

FIG. 4 illustrates a structure embodying the invention. Here an ARcoating covers the MQW modulator MOD and the surrounding photoresist PH.This structure is the result of the following steps.

Placing a drop of KOH solution on the surface of the exposed GaAs toremove any GaAs oxide.

Chemically removing the GaAs substrate SUB from the modulator MOD with ajet etcher by delivering a 1×1 mm jet of enchant onto the surface of thesubstrate SUB. The enchant is 100:1 H₂ O₂ :NH₄ OH, which stops on theAl₀.3 Ga₀.7 As layer LA1. The GaAs enchant does not attack thephotoresist appreciably nor Si or Al to the sides of the GaAs/AlGaAsmodulator. The enchant etches the substrate SUB in about 1.5 hours.

According to an embodiment of the invention, bond pads extend to theedge of the silicon device SD1 and the cement CM1 is applied withoutcoating them. According to another embodiment of the invention, the chipis wire-bonded and packaged before commencing the process.

After wire-bonding the Al pads of a modulator MOD, an SiOx AR-coating ARis deposited. The gold p contact CG serves as an integral reflector.

Yet another embodiment of the invention involves selectivephoto-chemical removal of the cement CM1 at the bond pads.

Another embodiment includes using a solvent to remove the photoresistlayers and the cement CM1 completely. This leaves the integrated deviceof Si chip and modulator MOD without the mechanical support of theetch-resist, but also without the mechanical burden of the substrateSUB.

According to an embodiment of the invention, the single modulator MODand the single connection to the Si device SD of FIGS. 1 to 4 representsbut one of a number of an array of modulators MOD. Each of the latter isgrown on a single substrate and bonded to corresponding contacts on thedevice SD with the single substrate SUB then removed.

FIG. 5 is a cross-sectional view illustrating a device integrating anumber of photonic elements with electronic elements of an IC chip andembodying aspects of the invention. Here, a number of modulators MOD,identical to the modulators MOD in FIG. 4, are bonded via bondedcontacts CN collectively representing the contacts CG, CO, COI1, COI2,COA1, COA2, CI1 and CI2 to the substrate SIS of a Si device SD. Thebonding process is the same as the process in FIGS. 2 to 4 except thatall the modulators MOD start on a single substrate SUB and the Si deviceincludes a number of conductor pairs each matching the conductor pair ofthe modulator MOD above that pair. Layer RL1 as well as cement CM1extend between and around the contacts CN and the modulators MOD. Asingle previously-removed substrate SUB for the modulators MOD appear inphantom lines. Layer RL1 as well as cement CM1 extend, with layer RL1 ontop of the cement CM1, between the substrate SIS and the level of theremoved substrate SUB.

FIG. 5 shows a single line of modulators MOD. The invention contemplatestwo dimensional arrays of such modulator MOD as shown in FIG. 6. Becauseoptical input/outputs (I/O's), such as the multiple quantum wellmodulators MOD, permit transmission and reception normal to the surfaceof the chip, such two-dimensional arrays offer substantial possibilitiesfor use in hybrid communication and information processing environments.

In operation, the output of an off-chip laser splits into an array ofspots and focuses on the multiple quantum well modulators MOD, whosereflectance is modulated by the on-chip electronics. This type of systemoffers the advantage of having a global clock (for oscillating thelaser). In addition, because such modulators are also efficientdetectors, one modulator can function as both receiver and transmitter.

Both the layer RL1 and the cement CM1 serve as an etch-resist, and astructural support for the assembled units. The cement used is onehaving a viscosity low enough to wick between the quantum wellmodulator, or modulators, MOD and the silicon device SD. For thispurpose, the cement CM1 preferably has a viscosity of less than onepoise when it is being applied. An example of a cement is an epoxy. Inpractice, epoxies tend to have lower viscosity at elevated temperatures.Thus the epoxy is applied while maintaining the modulator, ormodulators, MOD and the silicon device SD at an elevated temperature,such as 100° C. The latter temperature is lower than the melting pointof the solder on the modulator and silicon device. A preferred epoxy iscomposed of two components, and cures only after a delay followingmixing of the components. Such a delay permits application of the epoxybefore hardening. Any cement that satisfies the viscosity conditions andis resistant to the substrate-removal etch is suitable. This includescements that cure on the application of heat without emitting gas orliquid solvents and that set permanently. The term cement is used hereinto include epoxies and other cements. A preferred epoxy is availableunder the trademark Able Bond 931-1 manufactured by Able-StickLaboratories, a subsidiary of National Starch and Chemical Company,Rancho Dominguez, Calif. 90221. Such an epoxy has low viscosity at roomtemperature and is stored at -40° C.

FIG. 7 illustrates the manner of applying the cement CM1 to a structuresuch as shown in FIG. 6 and composed of a substrate SUB with a number ofmodulators MOD connected by contacts CN to the silicon device SD. Thelayer RL1 covers the substrate SUB and other portions of the modulatorMOD. In this example epoxy is used as the cement. However, othersuitable cements may be used. This process involves dipping an opticalfiber OF into an uncured epoxy, so as to place a bead BE of the epoxyonto the tip of the fiber. A micrometer stage MS holds the fiber OF andallows controlled movement in all directions. While the modulators MODand the silicon device SD are being viewed through a microscope MSP, themicrometer stage MS places the tip of the fiber OF onto the side of thesubstrate SUB near the silicon device SD so that the epoxy bead touchesthe corner or gap formed between the layer RL1 on the substrate and thesilicon device. The epoxy then wicks between the layer RL1 on thesubstrate SUB and the silicon device SD and around the modulators MOD.The modulators MOD and the silicon device SD rest on a heated platformHP to elevate their temperatures to 100° C. during application of theepoxy to reduce the viscosity further. Several epoxy beads may be wickedbetween the layer RL1 on the substrate SUB and the silicon device SDuntil the volume between them. The epoxy is then cured by placing themodulators MOD and the silicon device SD with the epoxy into an oven at100° C. for one hour.

Thereafter the substrate on the modulator MDD is removed. The cementCM1, e.g. the epoxy, is left on the device after the substrate SUB hasbeen removed from the modulators, MOD.

The invention furnishes a technique for solder-bonding one semiconductordevice onto another and removing the substrate from one. In particularthe invention provides a method of bonding GaAs/AlGaAs 850 nm λmodulators onto silicon. According to an embodiment of the inventionthis technique forms whole arrays of devices in one step. This techniqueprovides a method for photoelectronic integration of silicon IC's.

The invention enables the substrate of the optical GaAs/AlGaAs modulatorto be removed after it is solder-bonded to a silicon chip. Removal ofthe substrate is necessary since it is opaque to light at the wavelengthneeded for operation. In addition, substrate removal alleviatesmechanical constraints on the bond. An embodiment of the inventionincludes placing an etch-resist, for example an organic material such aspolyimide or benzocyclobutenes or photoresist, or an inorganic materialsuch as silicon nitride or silicon-oxy-nitride or silicon dioxide on theexposed surface of the substrate to be removed, and flowing cement CM1,such as an epoxy, between the chips to allow etching of the substrate.The flow may be enhanced by capillary action. The photoresist or epoxyprotects the front sides of the chips during etching and augmentsmechanical support. The technique can support fabrication of largearrays. Although simple, the invention permits the joining of complexelectronic circuits with optical inputs and outputs in large numbers.

An embodiment of the invention involves GaAs/AlGaAs p-i-n multiplequantum well modulators solder-bonded to a silicon substrate. The GaASsubstrate is chemically removed to allow operation at an opticalwavelength of 850 nm.

The invention promotes the use of photonics in an information processingenvironment where it is integrated with electronics. The invention takesadvantage of the greater capacity of electronics for complexity,functionality, and memory, and the greater capacity of photonics forcommunications. The photonic devices, such as the multiple quantum wellmodulators, function as optical interconnects between electronicintegrated circuit chips (IC's). The invention involves the integrationof photonic elements (both receiver and transmitter) on the IC chip. Ittakes advantage of the attractive feature of optical input/output (I/O)that it can occur normal to the surface of the chip, and allowtwo-dimensional arrays of interconnects to be formed, for surface-normalphotonic operation.

The invention takes advantage of silicon electronics's effectivetechnology where complex systems such as microprocessors or memory areconcerned. It offers the benefit of increased communication capacity tothe IC chip when the chip contains a great number of computing elements.

One of the advantages of GaAs/AlGaAs multiple quantum well modulators istheir typical operation at 850 nm. This short wavelength allows theformation of small optical spots whose potential spot sizes vary withthe wavelength.

The structure and process described in connection with FIGS. 1 to 4represent a single example of a integrated semiconductor device. Otherembodiments of the invention use other dimensions, particularly areadimensions, and different materials. For example any suitableetch-resist, that is any polymer that resists the enchant and that driesinto a mechanically sound solid material may be used for the layer RL1The term etch-resist as used herein can refer to any polymer that driesto protect an underlying solid from the enchant and includes aphotoresist and non-polymer layers such as silicon nitride or siliconoxides. A suitable etch-resist for use herein is one that becomessufficiently solid furnish mechanical support.

According to an embodiment of the invention, not only epoxy, but anyinsulating cement may be used where the epoxy is used as long as thecement's viscosity permits wicking between the substrates and, uponsolidifying, furnishes mechanical strength to the device and providesetch-resistance. The term cement is used herein generically to includeair drying thermoplastics, and thermosetting materials such as epoxies,acrylics, and urethanes. The term cured is used herein to meansolidified, and the term uncured refers to the cement in its liquidstate.

The term "insulating cement" refers to the cement in its solidified,i.e. cured, form. Preferably it is insulating in its liquid form alsobut may have non-insulating properties in liquid form.

Moreover the contacts on each semiconductor device need not be coplanar,as long as they complement the heights of the Si-mounted contacts towhich they are to be bonded. Also, as an example, the bonding materialneed not be indium (In). According to other embodiments of theinvention, the contacts are gold, or various mixtures of In, Au, Sn,and/or Pb.

Furthermore, the Si device SD need not be a Si IC chip. The Si device SDmay be any fully-fabricated semiconductor device such as one made ofGaAs. The invention prevents the damage to the semiconductor devicewhich would be caused by growing of one device on the otherfully-fabricated device. Moreover, the modulators MOD may have anystructure from which a substrate must be removed.

According to an embodiment, the deposition of the layer RL1 is in theform of chemical vapor deposition when inorganic material such assilicon nitride or silicon-oxy-nitride or silicon oxide is used. Inanother embodiment inorganic materials are deposited by coating inliquid form and cured.

While embodiments of the invention have been described in detail, itwill be evident to those skilled in the art that the invention may beembodied otherwise without departing from its spirit and scope.

What is claimed is:
 1. A method of forming an integrated semiconductordevice, comprising the steps of:placing an etch-resist upon a substrateand about conductors of first semiconductor device having a firstsubstrate and conductors. bonding the conductors of the firstsemiconductor device to conductors on a second semiconductor devicehaving a second substrate; flowing cement to fill a space between theetch-resist on the first semiconductor device and the secondsemiconductor device; allowing the cement to cure; and removing thesubstrate from the first semiconductor device.
 2. A method as in claim 1wherein the cement is an epoxy.
 3. A method as in claim 1, wherein thestep of flowing includes flowing the uncured cement between and aroundsaid conductors around said conductors on said second semiconductordevice.
 4. A method as in claim 1, wherein the step of bonding includesforming surfaces of any one of a group consisting of In, Au, andmixtures of In, Au, Sn, and Pb on the conductors of one of said firstsemiconductor device and said second semiconductor device.
 5. A methodas in claim 3, wherein the cement is an epoxy and the step of flowingincludes wicking the uncured cement between the etch resist on the firstsubstrate and the second substrate while the conductors of said firstsemiconductor device and said second semiconductor device are attemperatures above room temperature.
 6. A method as in claim 1, whereinthe step of flowing includes flowing sufficient cement so that saidcement and said etch-resist, when dried, form a structural support fromone of the semiconductor devices to the other.
 7. A method as in claim1, wherein the second semiconductor device is a photonic device and thefirst semiconductor device is a Si device.
 8. A method as in claim 1,wherein the first semiconductor device includes a modulator having aGaAs/AlGaAs multiple quantum well modulator unit and said secondsemiconductor device includes a Si integrated circuit chip.
 9. A methodas in claim 1, wherein said first semiconductor device includes aplurality of GaAs structures on said substrate of said firstsemiconductor device, said conductors on said first device including aplurality of terminals on each of said GaAs structures, said secondsemiconductor device includes a plurality of GaAs structures on saidsubstrate of said second semiconductor device, said conductors on saidsecond device including a plurality of terminals on each of said GaAsstructures;said step of placing an etch-resist includes placing anetch-resist on the substrate of said first structures and about theterminals of each of said structures on said first semiconductor device;said step of bonding the conductors includes bonding said terminals ofsaid first structures to said terminals of said second structures.
 10. Amethod as in claim 3, wherein the cement is an epoxy and the step offlowing includes wicking the uncured cement between the first and secondsubstrates at room temperature.